Fixes #4125

Make AssertJMultipleFailuresError#getMessage render the cause of each failure that has one, so SoftAssertions#fail(String, Throwable) shows cause message: ... and the first stack trace elements of the cause again, as in 3.23.1. Failures without a cause are formatted exactly as before.

The cause was lost because getMessage only used the failure message; Throwables.describeErrors (the #864 fix) was reached only through the SoftAssertionError fallback, which never runs when opentest4j is on the classpath.

SoftAssertionsErrorDescriptionTest did not catch this since it asserted the opentest4j prefix but not the cause; it now asserts the cause too. main is not affected: 452cee7 already renders the cause in MultipleAssertionsError.